Amine−Phenolate Ligands in Niobium Chemistry: π-Interactions Probed by an Ancillary Alkyne Ligand

A series of alkyne niobium complexes based on tetrapodal amine bis(phenolato) ligands NbCl[ONNO]R(MeCCMe) ([ONNO]R = N(CH2CH2NMe2)2(CH2C6H2-3,5-R2-2-O), R = H, Me) (4), and tripodal amine phenolato ligands NbCl2[ONN]Me(MeCCMe) ([ONN]Me = NMe(CH2CH2NMe2)(CH2C6H2-3,5-Me2-2-O)) (5) and NbCl2[ONOO]R(MeCCMe) ([ONOO]R = N(CH2CH2OMe)2(CH2C6H2-3,5-R2-2-O)) (R = tBu, Me) (6), have been synthesized and characterized by spectroscopic and X-ray studies. For complexes 4 and 5, two isomers differing by the alkyne position have been obtained, separated, and fully characterized. The alkyne ligand is either trans to the sidearm NMe2 group or to the tripodal nitrogen atom. The methyl derivatives of 4 and 5, NbMe[ONNO]H(MeCCMe) (7) and NbMe(Cl)[ONN]Me(MeCCMe) (8), respectively, have been synthesized. Throughout the series, structural and 13C NMR data testify to the competition between alkyne and phenolato orbitals for available d orbitals on the niobium, a rarely observed picture for group 5 metal complexes.